Signal-to-noise ratio (SNR) is an important parameter in the design and operation of communication systems. Communication signals take many forms within a communication system. These forms include a signal that has been modulated with user-supplied information and which is processed within a transmitter into an upconverted analog RF communication signal and an amplified RF communication signal that is broadcast from the transmitter. These forms also include an analog RF communication signal applied at the front-end of a receiver, and a downconverted communication signal applied to a demodulator in the receiver. Many of these forms of the communication signal are composite signals for which one portion of the signal power is a usable and desirable part that conveys the user-supplied information and another portion of the signal power is an unusable and undesirable part that impedes the extraction of the user-supplied information by the receiver. The signal part of the SNR is considered to be the useable portion of a communication signal while the noise part is considered to be the unusable portion. SNR is the ratio formed by dividing the signal part by the noise part. One of the goals of conventional communication systems has been to maximize the signal portion and minimize the noise portion throughout the communication system to achieve as high an SNR as reasonably possible for a given set of circumstances. Conventionally, a higher SNR can be translated into a higher link capacity, lower power consumption, more efficient spectrum usage, greater range of communication, and the like.
For a communication system, an SNR is typically relevant only for the in-band portion of the communication signal. But out-of-band considerations also play a role. When considering an amplified communication signal broadcast from an RF transmitter, the transmitter's amplifier is primarily responsible for providing the signal's power. But the amplifier may be prevented from providing the maximum amount of power it is capable of providing. The governmental regulatory agencies that license RF spectrum for use by RF transmitters define spectral masks with which transmitters should comply. The spectral masks set forth how much RF energy may be transmitted from the RF transmitters at various in-band and out-of-band frequencies. As transmitter technology has advanced, and as increasing demands have been placed on the scarce resource of the RF spectrum by the public, the spectral masks have become increasingly strict. In other words, very little energy outside of an assigned frequency band is permitted to be transmitted from an RF transmitter. Many popular modern modulation techniques, such as CDMA, QAM, OFDM, and the like, require the amplifier to perform a linear amplification operation. But any deviation from perfect linearity in the amplification process causes spectral regrowth, where the amplified signal includes more out-of-band RF energy than is present in the form of the communication signal presented to the amplifier for amplification. Regulatory spectral masks require the amplifier's linearity to be at a very high level so that very little of the signal's power appears outside the spectral mask.
One of the ways to achieve the high degree of linearity imposed by a spectral mask is through the use of an input power backoff controller. The input power backoff controller causes the communication signal being amplified in a transmitter's amplifier section to receive the maximum amount of amplification that the amplifying section can deliver at all times without causing a violation of the spectral mask. Thus, the backoff controller imposes an upper limit on the signal portion of the communication signal broadcast from the transmitter. But it also controls the amplifier section so that the amplifier adds very little in-band and out-of-band noise to the communication signal. Thus, the backoff controller may cause the signal level to rise and fall as it continuously adjusts to better maximize amplification without violating a spectral mask, but the noise contributed by the amplifier remains at such a low level that it satisfies the spectral mask both before and after any such adjustments.
Conventional transmitters occasionally include a peak controller which controls a peak-to-average-power ratio (PAPR) parameter of the communication signal that will subsequently be amplified before being broadcast from the transmitter. One example of a PAPR reduction circuit is described in U.S. Pat. No. 7,747,224, issued 29 Jun. 2010, and entitled “Method and Apparatus For Adaptively Controlling Signals”, which is incorporated by reference in its entirety herein. A PAPR reduction circuit like the one described in U.S. Pat. No. 7,747,224 and elsewhere, reduces the PAPR of the communication signal prior to amplification. Peak reduction is desirable because it allows the transmitter's amplifier to operate at a lower backoff point relative to average signal power. By operating at a lower average power backoff point, average signal power may be increased, for example through the operation of the backoff controller, thereby increasing the signal portion of the communication signal's SNR. Other advantages follow, including operating the amplifier at a greater level of power added efficiency (PAE), more link capacity, an ability to use a lower cost amplifier, efficiency improvements in the use of the spectrum, and the like.
But the benefits of peak reduction come at a cost. In particular, the peak reduction process introduces noise into the communication signal, and the amount of noise introduced increases as more peak reduction is achieved. The peak controller is desirably configured so that this noise is primarily located in-band and so that no spectral mask violations occur. Usually, meaningful amounts of peak reduction occur where the peak controller has introduced such a significant amount of noise that the noise contributed by the amplifier and other downstream sections of the transmitter may be ignored in SNR characterizations of the communication signal. This presents a situation where a transmitter's PAPR and backoff systems operating together bond signal and noise parameters of the communication signal together so that a decrease in SNR is accompanied by an increase in signal, and an increase in SNR is accompanied by a decrease in signal.
Industry-standardized specifications have been proposed and/or promulgated so that radio equipment manufacturers can know how to configure their equipment to successfully communicate with the equipment of other manufacturers. Such specifications set stringent noise specifications for communication signals broadcast from transmitters. The major standards set forth these specifications in the form of an error vector magnitude (EVM) specification or, for CDMA-based systems, a waveform quality factor (ρ) specification. A relative constellation error (RCE) metric is also defined, where RCE is nearly interchangeable with EVM but is given a different label due to the use of a different measurement technique.
EVM is often designated as:
  EVM  =            100      ×                                    N            T                    S                      =          100              SNR            where NT represents a transmitter's broadcast communication signal's noise power and S represents the signal power. The constant value of one-hundred is included so that EVM will be expressed as a percentage. Thus, EVM is a specification's way of characterizing essentially the same phenomenon that is expressed using the SNR parameter, i.e., a ratio between signal and noise, although it may be accompanied by a precise definition of a measuring procedure. EVM increases as SNR decreases. As a typical example, EVM may be set at around 17% for a QPSK modulation at rate ½ encoding. This is equivalent to an SNR of 34.6, or 15.4 dB when expressed in decibels. Different EVM values are specified for different specification-compliant modulation and coding parameters. The standards specify EVM/RCE to decrease (or equivalently for SNR to increase) as the modulation and coding parameters change to accommodate increased link capacity. The waveform quality factor ρ is a similar metric that is directly related to SNR and EVM. EVM may be expressed as:
  EVM  =                    (                  1          ρ                )            -      1      
Most conventional transmitters, including those with PAPR and backoff systems and the above-discussed U.S. Pat. No. 7,747,224, teach controlling their PAPR and backoff systems to rigidly and directly comply with the dictates of industry standard EVM, RCE, or p specifications. But one example of a transmitter with PAPR and backoff systems that complies with the dictates of industry standard EVM specifications in a less direct manner is described in U.S. Publication No. 2011/0064162, published 17 Mar. 2011, and entitled “Transmitting Unit That Reduces PAPR and Method Therefor”, which is incorporated by reference in its entirety herein. The transmitter of U.S. Publication No. 2011/0064162 teaches that the EVM specification is entirely appropriate for the various modulation and coding parameters that are set forth in the specification, but that in certain situations other optional encoding schemes may achieve coding gain relative to corresponding non-optional coding schemes. For example, an optional iterative encoding/decoding scheme or a block encoding scheme characterized by large amounts of latency may be usable in some situations to improve coding gain for the communication link. In these situations, the coding gain should permit a corresponding relaxation in the EVM specification to an alternate EVM point that achieves an equivalent SNR at the receiver. But the major industry standard specifications do not define any such alternate EVM point. Thus, U.S. Publication No. 2011/0064162 teaches operating a transmitter at a technically noncompliant EVM point to compensate for also operating with optional alternative coding parameters to achieve the same SNR result in the receiver that the specification achieves with non-optional coding parameters. In other words, it teaches operating with adherence to the spirit of the EVM specifications if not technically within the letter of the EVM specifications.
Conventional communication systems and transmitters have failed to appreciate that, in some situations honoring EVM, RCE, or p specifications, whether directly as taught in U.S. Pat. No. 7,747,224 or indirectly as taught in U.S. Publication No. 2011/0064162, actually harms communication link integrity even when other communication link parameters are compliant with the specifications. Harm results in two different ways. In some situations, receiver SNR may experience an undesirable decrease when operating a transmitter below EVM specifications (ie., at even less transmitter noise than is specified relative to signal) and a desirable increase when nevertheless operating above EVM specifications (ie., at more transmitter noise than is permitted relative to signal). This type of operation leads to lower link capacities than are readily achievable at the same power levels and costs and to a reduced efficiency in spectrum usage. Moreover, when operating in an SNR region that extends up to an equilibrium point, receiver SNR may decrease more the further below the EVM specifications a transmitter operates, and receiver SNR may increase more the further above the EVM specifications a transmitter operates. In other words, the major industry standards and the conventional equipment which are provided and/or proposed to comply with the major industry standards fail to appreciate the existence of any equilibrium point, where the equilibrium point is that signal-to-noise ratio (SNR) for the signal broadcast from the transmitter where a demodulator in a receiver will experience a reduced SNR if the transmitted signal SNR either increases or decreases.
And, in these situations, communication link robustness suffers, further harming communication link integrity. Communication links desirably remain operational near their link capacity at all times. Otherwise, the spectrum is not being used efficiently. But communication links operate in a dynamic environment that includes interference, fades, and switching between different coding and modulation formats. Link interference effects, fading effects, and coding/modulation switching discontinuity effects may all be worse when operating within mandated EVM specifications but could all be improved by operating outside mandated EVM specifications.
These situations occur where transmitters employ both PAPR controllers and backoff controllers. In these transmitters, since signal and noise are operationally bonded together, more operational transmitter noise yields a greater signal level, albeit at a lower SNR or higher EVM level. Unfortunately, industry standard specifications mandate operation in this region where communication link harm results from adhering to EVM or corresponding specifications.